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Review and Measurements of Massdrop THX AAA 789 Amp

Audiodoc

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It looks quite similar even in texture.
 

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Audiodoc

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I have the original Sennheiser HD650 and Hifiman Massdrop 4xx as my main headphones.
 

no1boss

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Just to verify, there's no problem plugging in powered speakers to this thing, correct? in my case SE bypass to Audioengine A5+.
 

Audiodoc

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I don't think that there will be any problem. The dictum in audio is to keep the path short and simple DAC to audioengine powered speakers. This amp may help if the output of you DAC is very low eg less than 2V or there is an impedance matching issue. There it will act as a preamplifier.
 
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amirm

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May I ask if there'll be a teardown of this unit :p?
This is a loaner unit and I don't have permission to open it. But I have bought one that is supposed to ship any day now. When I get it, I will open it.
 

sbsj

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omg :facepalm: ... Just got notified that this is on Massdrop again.

I have got two days to decide which to buy for my HD6XX ;
Massdrop THX AAA 789 or the JDS LABS ATOM amp :rolleyes:
 
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amirm

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I have got two days to decide which to buy for my HD6XX ;
Massdrop THX AAA 789 or the JDS LABS ATOM amp :rolleyes:
Both work very well with my HD-650. If your DAC has balanced output, then my preference would be the 789 as that deals with ground loops. Otherwise the atom is great and will leave you money for other holiday shopping. :)
 

Roen

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TheBatman_Yo

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Both work very well with my HD-650. If your DAC has balanced output, then my preference would be the 789 as that deals with ground loops. Otherwise the atom is great and will leave you money for other holiday shopping. :)

Any good recommendations for DACs with balanced output?
 

L-Train

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Hi all, I've been enjoying my 789 for a little while now and decided to do a few tests on my unit. One thing I found is a strange rise in crosstalk vs. frequency when the gain is set to its lowest position (1). Here's a screenshot of what I'm seeing:

crosstalk.png


For gains 2 and 3, crosstalk is practically identical to the soundcard's loopback as expected, but for gain 1, it rises steadily above ~100 Hz. Note that these tests were done unloaded for the best possible results.

Here's my measurement setup:
  • Lynx Studio E44 soundcard (balanced output/input) at 96 kHz, 24 bit
  • THX AAA 789 (3.5mm SE output -> 3.5mm to RCA adapter -> RCA to XLR cables wired as per LynxTWO manual)
    • Results are identical with the 1/4" SE output
  • Volume set to maximum for all tests except for the test at gain 1 (volume set to ~4 o'clock to reach 6 dBV/2 Vrms)
  • RightMark Audio Analyzer 6.4.5 software
Other than crosstalk, all other unloaded measurements (frequency response, noise, THD, IMD, etc.) match the soundcard's loopback at all gain settings.

@amirm / @AndrewMason , can you see if you're able to replicate this? I find it interesting that this only happens at the lowest gain setting, and since I use that setting for normal listening I'm a tiny bit disappointed that it doesn't perform quite as well in this aspect, but of course with a single-ended headphone load this is probably inaudible. I don't have the connectors to build an XLR-4 to dual XLR-3 adapter right now, but it'd also be interesting to see if this rise in crosstalk at gain 1 also happens with the balanced output.
 

graz_lag

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Any good recommendations for DACs with balanced output?

KORG DS-DAC100
I bought it for $190 from Amazon
XLRs and RCAs, fixed levels
only USB, no Toslink no SPDIF
Headphones amp with volume control (it drives my DT-880 250 Ohms @ 40% of the scale; my AKG K701 @ 55% of the scale)
Gets the power from the USB, thus it can be powered by a 5V external battery
It comes with it's audio software (AudioGate), it plays all music formats, including DSD natively
Only minus : not seen by Linux, only Windows
 
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amirm

amirm

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@amirm / @AndrewMason , can you see if you're able to replicate this? I find it interesting that this only happens at the lowest gain setting, and since I use that setting for normal listening I'm a tiny bit disappointed that it doesn't perform quite as well in this aspect, but of course with a single-ended headphone load this is probably inaudible. I don't have the connectors to build an XLR-4 to dual XLR-3 adapter right now, but it'd also be interesting to see if this rise in crosstalk at gain 1 also happens with the balanced output.
Sure. Have to run out in a few minutes but will do it later. Remind me if I forget.
 

rajapruk

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AndrewMason wrote in a Massdrop-comment that all TRS-jacks has some inherent crosstalk, if I remember correctly. And that xlr-4 is the way to go for the least amount of crosstalk from the amp.
 

L-Train

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AndrewMason wrote in a Massdrop-comment that all TRS-jacks has some inherent crosstalk, if I remember correctly. And that xlr-4 is the way to go for the least amount of crosstalk from the amp.

Ah yes I remember that comment. Unfortunately I believe this is a separate issue since it doesn't appear on the gain 2/3 crosstalk tests which also use the TRS jacks (tested on both the 3.5mm and 1/4" jacks).
 

derp1n

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index.php

:cool:
 
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amirm

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With distortions as low as what we see in THX AAA, you can't just use hobby level signal generators. Certainly not one using a DAC. That may very well be the limiting fact, which helps equalize the two devices.

Here is the 1 Khz FFT that I ran:

1542508006472.png


As we see, the distortion spikes are at -130 dB whereas he shows it at -115 or so. Furthermore, the amplitude signature of the distortion spikes in my measurements don't match his.

He adds these notes to his graph:
1542508109340.png


#2 is completely wrong. The "-140 dB" numbers seen on the FFT graphs is not in any form or fashion the actual noise floor of the equipment. Depending on how many points are used in FFT, the gain (reduction in noise level) can be over 30 dB! This is basic signal processing/measurement things that needs to be known before one advocates any such data.

In addition, I doubt that his measurement gear has low enough noise/noise modulation to be of much use here.

Hobby level measurements like he is doing are OK at much more gross levels of distortion and noise. But when we are measuring state-of-the-art gear like THX, you better darn well use state-of-the-art measurement gear whose own performance is superior to the device being tested. Otherwise the performance of the measurement gear dominates causing #4: "distortion characteristics at 1 kHz look extremely similar." Well, maybe they do because in both cases it is the distortion of the measurement gear you are looking at, not that of the THX Amp!
 
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